Self-aligned Process for SiC Power Devices
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1246-B06-05
Self-aligned Process for SiC Power Devices Tomoko Borsa1,2 and Bart Van Zeghbroeck1,2 TrueNano Inc., 4699 Nautilus Court, Suite 203, Boulder, CO 80301-4305, U.S.A. 2 Department of Electrical, Computer and Energy Engineering, University of Colorado at Boulder, 425 UCB, Boulder, CO 80309-0425, U.S.A.
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ABSTRACT A novel self-aligned process for the fabrication of SiC power devices and specifically BJTs (bipolar junction transistors) has been developed. Due to its excellent material properties, SiC BJTs have great potential for many power applications. For fast switching applications, it is critically important for BJTs to have small lateral dimensions for emitter stripes, base contact and a tight alignment between both. The desired fabrication technology calls for robust selfaligned processes that provide the needed low base resistance and a reliable and SiC-compatible contact metallurgy. The newly developed process incorporates electroless nickel plating of the emitter metal to form a sacrificial lift-off overhang structure, which is used to form a self-aligned contact relative to the edge of the emitter mesa. It enables a well controlled overhang independent of the SiC ridge profile and the height of the emitter mesa. This overhang structure was demonstrated on 4H SiC substrates fabricated by the novel self-aligned process enabling the fabrication of the design with high yield. INTRODUCTION Silicon carbide is a semiconductor with desirable material properties, such as a wide bandgap, high thermal conductivity, high breakdown field, and high saturated electron velocity [1]. Due to these properties, it is an excellent material for constructing power switching devices operating in harsh environments where conventional semiconductors cannot adequately perform [2]. One example of such a SiC power device is the 4H-SiC bipolar junction transistor. It is excellent for power switching applications due to the higher current/power handling and faster switch on/off times [3]. While the potential of the SiC BJT is recognized, appropriate techniques for producing devices are lacking due to its fabrication difficulty. Because SiC has such a strong chemical bonding, standard Si processing technology is not readily applicable to this material. For example, in order to achieve a high voltage 4H-SiC BJT switch with nanosecond switching time, the device must have a low base resistance. The simulation results indicate that for an emitter width of 2.0 Pm and a base width of 1.2 Pm the distance between the two should be 0.4 Pm or less to meet the requirement for base resistance. To produce the above-described geometries and spacing, it is desirable to construct the device in a self-aligned manner. Selfalignment in this context means that the relative spacing of features of the device, such as contacts, is automatically controlled by the processing sequence and process parameters, rather than by the careful alignment prior to exposure of a photo sensitive layer. Such novel selfaligned process for SiC BJT devices, enabling the
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